1. Field of the invention
The present invention relates to a power steering system for assisting steering force of wheels, and relates to a method of driving rear wheels with an effect of reducing a steering torque when steering a steering wheel during stoppage of a vehicle.
2. Description of related art
As a power assist device for assisting the force for steering the wheels in accordance with the steering torque input by a driver, a device using a hydraulic mechanism is general. As an example of this prior art, there is the one in which a hydraulic pressure is generated by driving an oil pump with an electric motor, and the generated hydraulic pressure is controlled by feeding back and controlling the steering torque input from the steering wheel to generate the steering assist force (see JP-A-2003-212141, for example).
In addition, an electric power steering system for assisting the steering only by an electric motor without using a hydraulic mechanism has come to be widely used, mainly in a compact car (see JP-A-8-295257, for example).
Further, there is known an art relating to a hybrid electric hydraulic power steering device capable of realizing energy saving and space saving by using an oil pump capable of discharging an operating oil in both directions to drive a motor on demand (see JP-A-2000-168604, for example).
It is generally known that the steering force in the case of steering the steering wheel when the vehicle is stopping is larger than that in a state where the vehicle is running. Accordingly, the maximum assist force which should be generated by the power steering system is specified by the steering assist force required at the time of stoppage of the vehicle (see “Automobile Steering System and Driving Stability” edited by Kayaba Industry Co., Ltd., P. 213, Sankaido Publishing Co., Ltd., for example).